by When we talk about the greenhouse effect (other than global warming), we rarely talk about the fact that most of it is not caused by carbon dioxide.
But, as a greenhouse gas, water vapor provides 70% of the insulation that keeps the sun’s energy within our atmosphere, at the right temperature for life.
The impact of CO2 is only about 20% of that, although it is rising. One of the reasons for this skewed view is that we don’t struggle to measure how much water vapor is around at any one time because it varies depending on where you are.
Meanwhile, we can measure CO2 everywhere, very accurately, because it is homogeneous and diffused throughout the globe.
We can only count what we can measure. However, measuring the most significant impacts may not be easy.
Warming is accelerating, already 1.5C above pre-industrial levels in some parts of the world, and many climate scientists worry that the models may be missing something.
Could the answer lie in a difficult and forgotten, but crucial piece of climate science?
Water cycles, big and small
Water cycles are extremely complex, with endless feedback loops, involving people, plants, animals, soil, weather and climate.
Modeling them is a huge challenge for most climate modellers, shying away from clear outputs for policy makers. In contrast the effect of CO2 is simple, and aesthetically boring, but easier to explain.
We all remember water cycles from geography classes. When mountains force a wet flow of air from the ocean up at mountains, it condenses, forms rain, and flows back into the sea via rivers.
This is the “big” water cycle. But in important parts of the world, like the Amazon or the Congolese rainforests, most of the rain comes from vegetation, the “small” water cycle.
Plants draw water from the soil and “sweat” out through their leaves during photosynthesis.
A large tree can cycle a lot of water this way – up to 100s of liters a day – but every plant contributes. They also release microscopic particles that “seed” the rain; Note that condensed water vapor can condense around and form raindrops.
No plants, no rain. Water begets water, say hydrologists; the womb is soil, the vegetation is the midwife.
Rain on our parade
How water affects global temperatures is complex. It takes energy as heat to make water vapor.
As plants emerge, it cools the surrounding air at ground level. In the air, the steam works to prevent infrared radiation coming from the earth’s surface, keeping heat in and warming everything.
But when it reaches higher into the atmosphere and condenses, it releases heat again. Because the air is thinner it is much easier for the heat to escape into space and the overall cooling effect becomes again.
This is called the biotic pump; vegetation takes heat from the ground and pumps it into space.
The cooling power of the plants can also influence major water cycles. Wet air from the sea in the Pacific Northwest, say, passes over land thickly covered with perspiring trees, becoming wetter and cooler as it goes. Even before it reaches the mountains it cools and precipitates enough to condense and rain.
Without the tree cover the vapor in that air can only condense when it hits the mountains, and then partially.
Wet air could then move over the plains behind the mountains, creating new (and likely some damaging) weather patterns as it goes.
Man-made feedback loops are to blame
Perhaps you will start at the relevance to human global warming calculations. Along with pollution, worsening weather, CO2 emissions and the collapse of biodiversity, the destruction of vegetation for agriculture is another key feature of the Anthropocene, creating progressive deserts.
We’ve been doing it for thousands of years. In Roman times, much of North Africa was fertile forest and grassland.
Spain and California were also heavily forested, in the case of southern Spain bare land, intensive farming, greenhouses and depleted water tables have now replaced forests.
Everywhere, we are breaking small water cycles, creating nasty feedback loops. Water vapor is still in the air, able to condense and release heat, driving local and global warming.
The resulting droughts can harm farmers, and the dried vegetation that remains is a fire starter. Exposed crusted soil cannot hold water when it rains, causing flooding.
Hydrologists call this point and state the cycle of drought, fire, flood: California is familiar with this. Left alone, the result is deserts, and the rain disappears completely.
Climate models are largely based on extrapolating fossil fuel emissions. Make no mistake, these effects are huge, and the mechanisms are clear.
But is there a critical missing factor connected to plants and water making things worse?
If so, this is great news because it gives us another axis to act on. Weaning ourselves off fossil fuels will take a generation. But as a dominant species, humans can repopulate the planet much faster than that.
Eurof Uppington is the CEO and Founder of Amfora, a Swiss-based importer of extra virgin olive oils.
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